This invention relates to fuel cell power production systems and, in particular, to a fuel cell power producing gas separation system and method.
A fuel cell is a device which directly converts chemical energy stored in a fuel into electrical energy through electrochemical reactions. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten Carbonate Fuel Cells (MCFCs) operate by passing a reactant fuel gas through the anode, while oxidizing gas, such as carbon dioxide and oxygen, is passed through the cathode.
Combustion-based power plants produce energy by combusting flammable hydrocarbon based fuels including coal, natural gas, biogas, and syngas. As a result of the combustion process, combustion-based power plants generate flue gas, which is often disposed of by atmospheric emissions. Such emissions, however, are harmful to the environment because they contain carbon dioxide which contributes to global climate change.
Accordingly, a number of approaches have been used to control or limit carbon dioxide emissions from combustion-based power plants. However, separating the carbon dioxide from the post-combustion flue gas is not cost effective because of the significant loss of energy (power and/or heat) as the result of application of carbon dioxide capture systems.
A system is disclosed in U.S. Pat. No. 5,232,793 in which carbon dioxide emissions in the flue gas of a fossil fuel power plant are reduced using a carbonate fuel cell in tandem with the power plant. In this system, the flue gas is added to an oxidant supply and the combined gas is used as the feed gas for the cathode of the molten carbonate fuel cell. The electrochemical reaction in the fuel cell then results in the carbon dioxide in the feed gas being transferred from the cathode to the anode of the fuel cell. The anode exhaust gas thus becomes concentrated with carbon dioxide gas. This permits the carbon dioxide gas to be separated from the flue gas. The system of the '793 patent uses an externally reforming fuel cell system. In this system, the anode exhaust gas still has substantial amount of hydrogen in addition to quantities of carbon monoxide, methane and water. A great portion of the water can be condensed, however, the '793 patent does not offer a cost effective method for separating the remaining hydrogen, methane and carbon monoxide from the anode exhaust. Purification of carbon dioxide in the anode exhaust stream by removing hydrogen, carbon monoxide and methane is required prior to sequestration or conversion of carbon dioxide to a useful form.
An integrated power production system is disclosed by U.S. Pat. No. 7,396,603 in which a fossil fuel power plant for processing fossil based fuel is arranged in tandem with a carbonate fuel cell having an anode and a cathode. In the system of the '603 patent, the flue gas of the power plant serves as the inlet gas for the cathode section of the fuel cell. In addition, the system of '603 uses internal reforming carbonate fuel cells, which reduces the amount of methane within the anode exhaust to a negligible amount. However, the anode exhaust gas leaving the anode section of the fuel cell contains substantial amount of hydrogen, carbon monoxide and water impurities. The anode exhaust is subjected to processing including purification and capturing of the carbon dioxide in the exhaust gas using a carbon dioxide separator. However, the integrated power production system of the '603 patent does not recover heat or useful gases present in the flue gas, and does not disclose the details of CO2 separation from the anode exhaust.